OK, #lightcube now exists in the page but the image location provided returns a 404 error.
You either have the wrong path, the wrong file name, or you have moved/deleted your image by mistake.
Turns out, after some poking around, that your path was wrong. You had this:
when it should have been this:
(note that one period has been removed from the front of the relative file path)
You just need to update your CSS with that change and your arrow will appear as intended.